1. Field of the Invention
This invention relates to tissue supporting devices, preferably vascular stents for repairing blood vessels and, more particularly, to removable devices which will temporarily and, if desired, permanently support a dilated stenosis of a tubular organ (hollow viscus) such as a blood vessel.
2. Description of the Prior Art
In the past, permanent or biodegradable devices have been developed for implantation within a body passageway to maintain vascular patency. These devices are typically characterized by the ability of such an intravascular device to be enlarged radially after having been introduced percutaneously, to be transported transluminally, and to be positioned in a desired location. These devices are either expanded mechanically, such as by the expansion of a mandrel positioned inside the device, or are capable of releasing stored energy to expand themselves upon actuation within the body.
Biodegradable stent-like members using suture materials in braided tubing or the like have been used as vascular stents. Unfortunately, such devices are limited in strength and application and require a relatively long period of time to dissolve.
U.S. Pat. Nos. 4,739,762, 4,776,337 and 4,733,665 disclose expandable and deformable intraluminal vascular grafts in the form of thin-walled tubular members which are expanded radially outwardly into contact with a body passageway, the members being plastically deformed beyond their elastic limit and the members being permanently fixed within the body. Suitable materials for the fabrication of these tubular-shaped members would include silver, tantalum, stainless steel, gold, titanium, or other suitable plastic materials which may be permanently deformed. Permanent deformation is achieved when the material is subjected to a force which is greater than the elastic limit of the material which is utilized to make the tubular member. The open-mesh configuration of such devices is soon encapsulated by body tissue and cannot be removed. The exceeding of the elastic limit of the material used in such devices is also believed to compromise the performance of the devices in situ.
U.S Pat. No. 4,969,458 discloses a vascular stent formed from a wire component made of material, such as copper alloy, titanium, or gold, wherein the wound configuration unwinds upon expansion and becomes a permanent prosthesis stent, similar to prior art devices disclosed above, and is, unfortunately, not removable.
U.S. Pat. No. 4,969,890 discloses various configurations of shape-memory alloy members which have been previously radially compressed and which, upon positioning within the body and thermal activation, expand by themselves to become a permanent prosthesis within the body. In this regard, the reference teaches a device which operates in a similar fashion to the device disclosed in U.S. Pat. No. 4,485,816. U.S. Pat. No. 4,485,816 discloses a shape-memory alloy staple which, when heated, penetrates and cinches tissue together. Shape-memory alloy historically has been used to perform work in such a fashion wherein the component remains in a strong austenitic state after temperature activation. That is, above its transition temperature from martensite to austenite, and as the references above disclose, the shape-memory alloy either dilates an incompetent blood vessel or holds segments of tissue together. Unfortunately, neither of these devices is practically removable.
It would therefore be advantageous to have a tissue supporting device that could be inserted into the body while in a dimensionally compact configuration and deformed mechanically and elastically into position while remaining in that deformed configuration to perform a function such as radial support, engagement with a thrombus, etc., the device remaining somewhat flexible to accommodate movement of soft tissue, the device further being subsequently removable. The subject invention provides such a device using shape-memory alloy in ways different, essentially the opposite, from those taught in the prior art.